Electronchemical deposition of tantalum and/or copper in ionic liquids

ABSTRACT

The invention relates to a process for the electrochemical deposition of tantalum and/or copper on a substrate in an ionic liquid comprising at least one tetraalkylammonium, tetraalkylphosphonium, 1,1-dialkylpyrrolidinium, 1-hydroxyalkyl-1-alkylpyrrolidinium, 1-hydroxyalkyl-3-alkylimidazolium or 1,3-bis(hydroxyalkyl)imidazolium cation, where the alkyl groups or the alkylene chain of the hydroxyalkyl group may each, independently of one another, have 1 to 10 C atoms.

The invention relates to a process for the electrochemical deposition of tantalum and/or copper on a substrate in an ionic liquid comprising at least one tetraalkylammonium, tetraalkylphosphonium, 1,1-dialkylpyrrolidinium, 1-hydroxyalkyl-1-alkylpyrrolidinium, 1-hydroxyalkyl-3-alkylimidazolium or 1,3-bis(hydroxyalkyl)imidazolium cation, where the alkyl groups or the alkylene chain of the hydroxyalkyl group may each, independently of one another, have 1 to 10 C atoms.

Tantalum is a platinum-grey, hard, very tough, elastic, extensible, polishable metal which can be rolled and forged. In air, it becomes covered with a protective oxide layer or is spontaneously oxidised by water. Thin tantalum layers can be used in a multiplicity of applications, for example as barrier, protective or sealing layers, which may also be an interlayer, for tank linings, (micro)electronic components or devices such as, for example, of tantalum electrolyte capacitors, in the production of incandescent wires or gold bond wires, magnetic recording media or thermal printing heads for ink-jet printers.

In surgery, tantalum is used as material for bone nails, bone substitutes, joint implants, clamps, jaw screws and other instruments since this metal of high atomic number has high biocompatibility and haemocompatibility, similar to titanium. Implants frequently consist of implant materials which are subsequently coated with a thin layer of tantalum (Dresdner Transfer-brief, 04/2001 edition, Volume 9, pub. TU Dresden, BTI—Beratungsgesellschaft für Technologietransfer and Innovationsförderung mbH, Technologie Zentrum Dresden Ion Treatment of Vascular Stents Increases Haemocompatibility and X-ray Contrast or Arzte-Zeitung of 17.04.2002, A Universal Type of Prosthesis—That's Old Hat).

Copper is a corrosion-resistant noble metal which has excellent electrical conductivity and thermal conductivity and also exhibits very low electromigration behaviour. In chip technology, thin layers of copper have been used for some years instead of the aluminium used earlier as contact material for the semiconductor structures.

For the application of thin layers to substrates, some physical and chemical vapour-deposition methods are known to the person skilled in the art, for example the sputtering method or vacuum vapour-deposition method. The electrochemical deposition of copper from an aqueous medium is likewise known (A. Thies, Galvanotechnik, 11 (2002)2837-2843). In the electrochemical deposition of copper onto tantalum in aqueous medium, as is desired in the so-called damascene process, in which a silicon chip is covered with a thin tantalum layer with a thickness of 20-70 nm by vapour deposition, and the copper contacting is then applied in aqueous medium, the problem arises that tantalum is spontaneously oxidised by water before the copper deposition takes place. This gives rise to not inconsiderable contact resistances between copper and the tantalum oxidised on the surface.

Owing to its reactive character, tantalum cannot, in contrast to copper, be deposited in aqueous media. Organic solvents are excluded owing to the risk of explosion and the problem of rendering them anhydrous.

Electrochemical methods for the deposition of tantalum in high-temperature salt melts, such as LiF/NaF/CaF₂ melts, at 500° C. (Mehmood et al., Materials Transactions, 44 (2003), 1659-1662) or from the mixture of K₂TaF₇ in, for example, the eutectic mixture LiF/NaF/KF (50/30120) at temperatures of 600-900° C. onto iron (JP H06-57479) are known. The extremely high temperatures and the corrosive behaviour of the high-temperature salt melts mean that these methods are unsuitable for some applications, for example for use in chip technology, or are uneconomic owing to the safety aspect during performance of the deposition and the high costs.

JP 2001279486 describes an electrochemical process for the deposition of tantalum in which the deposition is carried out in a molten salt which consists of tantalum pentachiloride, alkylimidazolium chloride and fluorides of an alkali metal or alkaline earth metal. The deposition is preferably carried out from the TaCl₅, LiF and 1-ethyl-3-methylimidazolium chloride system in the ratio 30 mol:60 mol:10 mol at temperatures of about 100° C. On repetition of this procedure, it was found that a pure element has not deposited, but instead a large amount of chloride was always present.

The object of the invention was to find an alternative method for the electrochemical deposition of tantalum and/or copper under anhydrous conditions.

The object is achieved by the process according to the invention.

The invention relates to a process for the electrochemical deposition of tantalum and/or copper on a substrate in an ionic liquid comprising at least one tetraalkylammonium, tetraalkylphosphonium, 1,1-dialkylpyrrolidinium, 1-hydroxyalkyl-1-alkylpyrrolidinium, 1-hydroxyalkyl-3-alkylimidazolium or 1,3-bis(hydroxyalkyl)imidazolium cation, where the alkyl groups or the alkylene chain of the hydroxyalkyl group may each, independently of one another, have 1 to 10 C atoms.

Tantalum and copper are deposited independently of one another on a very wide variety of substrates in a very wide variety of applications. However, tantalum and copper can also be deposited one after the other, as is desired in the particular application of chip technology, i.e. firstly tantalum is deposited electrochemically on silicon, for example a silicon wafer, using the method according to the invention, and subsequently copper is deposited onto the tantalum-coated silicon in the same medium. The disadvantage of the tantalum vapour-deposition methods from the prior art, that the corners and edges of the silicon wafer do not correspond to the requisite tantalum layer thickness of 20 nm, can be overcome by the method according to the invention.

The ionic liquids comprising at least one tetraalkylammonium, tetraalkylphosphonium, 1,1-dialkylpyrrolidinium, 1-hydroxyalkyl-1-alkylpyrrolidinium, 1-hydroxyalkyl-3-alkylimidazolium or 1,3-bis(hydroxyalkyl)imidazolium cation, where the alkyl groups or the alkylene chain of the hydroxyalkyl group may each, independently of one another, have 1 to 10 C atoms, which are suitable for the process according to the invention have high conductivity and are generally thermally stable up to 400° C. They have, for example, a broad electrochemical window in the cathodic branch which extends from −2000 mV to −3500 mV against ferrocene/ferrocinium, preferably from −2700 mV to −3000 mV against ferrocene/ferrocinium.

An alkyl group having 1 to 10 C atoms is taken to mean, for example, methyl, ethyl, isopropyl, propyl, butyl, sec-butyl or tert-butyl, furthermore also pentyl, 1-, 2- or 3-methylbutyl, 1,1-, 1,2- or 2,2-dimethylpropyl, 1-ethylpropyl, hexyl, heptyl, octyl, nonyl or decyl. The alkyl groups may also be partially or fully substituted by fluorine. Fluorinated alkyl groups are, for example, difluoromethyl, trifluoromethyl, pentafluoroethyl, pentafluoropropyl, heptafluoropropyl, heptafluorobutyl or nonafluorobutyl.

A hydroxyalkyl group having 1 to 10 C atoms is taken to mean, for example, 1-hydroxymethyl, 2-hydroxyethyl, 3-hydroxypropyl, 4-hydroxybutyl, furthermore also 5-hydroxypentyl, 6-hydroxyhexyl, 7-hydroxyheptyl, 8-hydroxyoctyl, 9-hydroxynonyl or 10-hydroxydecyl. The alkylene chain of the hydroxyl group may also be partially or fully substituted by fluorine. Fluorinated hydroxyalkyl groups can be described, for example, by the sub-formula —(CHF)_(n)—OH or —(CF₂)_(n)—OH, where n can denote 1, 2, 3, 4, 5, 6, 7, 8, 9 or 10.

Suitable anions which satisfy the above-mentioned condition in combination with the cations according to the invention can be selected from the group of perfluoroalkylsulfonate, perfluoroacetate, bis(fluorosulfonyl)imide, bis(perfluoroalkylsulfonyl)imide, tris(perfluoroalkyl)trifluorophosphate, bis(perfluoroalkyl)tetrafluorophosphate, tris(perfluoroalkylsulfonyl)methide or perfluoroalkylborate.

The term perfluoroalkyl group means that all H atoms of the corresponding alkyl group have been replaced by F atoms. The perfluoroalkyl groups in the anions indicated preferably each have, independently of one another, 1 to 10 C atoms, particularly preferably 1, 2, 3 or 4 C atoms.

Anions which are suitable in accordance with the invention can be selected, for example, from the group of trifluoromethylsulfonate, pentafluoroethylsulfonate, heptafluoropropylsulfonate, nonafluorobutylsulfonate, bis(fluorosulfonyT)imide, perfluoroacetate, bis(trifluoromethylsulfonyl)imide, bis(pentafluoroethylsulfonyl)imide, bis(heptafluoropropylsulfonyl)imide, bis(nonafluorobutylsulfonyl)imide, tris(trifluoromethylsulfonyl)methide, tris(pentafluoroethylsulfonyl)methide, tris(heptafluoropropylsulfonyl)methide, tris(nonafluorobutylsulfonyl)methide, tris(pentafluoroethyl)trifluorophosphate, tris(heptafluoropropyl)trifluorophosphate, tris(nonafluorobutyl)trifluorophosphate, bis(pentafluoroethyl)tetrafluorophosphate, tetrakis(trifluoromethyl)borate, tetrakis(pentafluoroethyl)borate, trifluoromethyltrifluoroborate, pentafluoroethyltrifluoroborate, bis(trifluoromethyl)difluoroborate, bis(pentafluoroethyl)difluoroborate, tris(trifluoromethyl)fluoroborate, tris(pentafluoroethyl)fluoroborate or bis(pentafluoroethyl)trifluoromethylfluoroborate.

As soon as a plurality of perfluoroalkyl groups occur in the anions, these can denote, independently of one another, different perfluoroalkyl groups. The definition given above therefore also includes, for example, mixed anions, such as trifluoromethylsulfonylpentafluoroethylsulfonylimide, bis(trifluoromethyl)sulfonylpentafluoroethylsulfonylmethide.

The anions trifluoromethanesultonate, bis(trifluoromethylsulfonyl)imide or tris(pentafluoroethyl)trifluorophosphate are particularly preferably selected.

Suitable cations are, optionally linear or branched, tetramethylammonium, tetraethylammonium, tetrapropylammonium, tetrabutylammonium, tetrapentylammonium, tetrahexylammonium, tetraheptylammonium, tetraoctylammonium, tetranonylammonium, tetradecylammonium, trimethylalkylammonium, trimethyl(ethyl)ammonium, triethyl(methyl)ammonium, trihexylammonium, methyl(trioctyl)ammonium, tetramethylphosphonium, tetraethylphosphonium, tetrapropylphosphonium, tetrabutylphosphonium, tetrapentylphosphonium, tetrahexylphosphonium, tetraheptylphosphonium, tetraoctylphosphonium, tetranonylphosphonium, tetradecylphosphonium, trihexyltetradecylphosphonium, triisobutyl(methyl)phosphonium, tributyl(ethyl)phosphonium, tributyl(methyl)phosphonium, 1,1-dimethylpyrrolidinium, 1-methyl-1-ethylpyrrolidinium, 1-methyl-1-propylpyrrolidinium, 1-methyl-1-butylpyrrolidinium, 1-methyl-1-pentylpyrrolidinium, 1-methyl-1-hexylpyrrolidinium, 1-methyl-1-heptylpyrrolidinium, 1-methyloctylpyrrolidinium, 1-methyl-1-nonylpyrrolidinium, 1-methyl-1-decylpyrrolidinium, 1,1-diethylpyrrolidinium, 1-ethyl-1-propylpyrrolidinium, 1-ethyl-1-butylpyrrolidinium, 1-ethyl-1-pentylpyrrolidinium, 1-ethyl-1-hexylpyrrolidinium, 1-ethyl-1-heptylpyrrolidinium, 1-ethyl-1-octylpyrrolidinium, 1-ethyl-1-nonylpyrrolidinium, 1-ethyl-1-decylpyrrolidinium, 1,1-dipropylpyrrolidinium, 1-propyl-1-butylpyrrolidinium, 1-propyl-1-pentylpyrrolidinium, 1-propyl-1-hexylpyrrolidinium, 1-propyl-1-heptylpyrrolidinium, 1-propyl-1-octylpyrrolidinium, 1-propyl-1-nonylpyrrolidinium, 1-propyl-1-decylpyrrolidinium, 1,1-dibutylpyrrolidinium, 1-butyl-1-pentylpyrrolidinium, 1-butyl-1-hexylpyrrolidinium, 1-butyl-1-heptylpyrrolidinium, 1-butyl-1-octylpyrrolidinium, 1-butyl-1-nonylpyrrolidinium, 1-butyl-1-decylpyrrolidinium, 1,1-dipentylpyrrolidinium, 1-pentyl-1-hexylpyrrolidiniu m, 1-pentyl-1-heptylpyrrolidinium, 1-pentyl-1-octylpyrrolidinium, 1-pentyl-1-nonylpyrrolidinium, 1-pentyl-1-decylpyrrolidinium, 1,1-dihexylpyrrolidinium, 1-hexyl-1-heptylpyrrolidinium, 1-hexyl-1-octylpyrrolidinium, 1-hexyl-1-nonylpyrrolidinium, 1-hexyl-1-decylpyrrolidinium, 1,1-dihexylpyrrolidinium, 1-hexyl-1-heptylpyrrolidinium, 1-hexyl-1-octylpyrrolidinium, 1-hexyl-1-nonylpyrrolidinium, 1-hexyl-1-decylpyrrolidinium, 1,1-diheptylpyrrolidinium, 1-heptyl-1-octylpyrrolidinium, 1-heptyl-1-nonyl-pyrrolidinium, 1-heptyl-1-decylpyrrolidinium, 1,1-dioctylpyrrolidinium, 1-octyl-1-nonylpyrrolidinium, 1-octyl-1-decylpyrrolidinium, 1-1-dinonylpyrrolidinium, 1-nonyl-1-decylpyrrolidinium or 1,1-didecylpyrrolidinium, 1-hydroxymethyl-1-methylpyrrolidinium, 1-hydroxymethyl-1-ethylpyrrolidinium, 1-hydroxymethyl-1-propylpyrrolidinium, 1-hydroxymethyl-1-butylpyrrolidinium, 1-(2-hydroxyethyl)-1-methylpyrrolidinium, 1-(2-hydroxyethyl)-1-ethyl pyrrolidinium, 1-(2-hydroxyethyl)-1-propylpyrrolidinium, 1-(2-hydroxyethyl)-1-butylpyrrolidinium, 1-(3-hydroxypropyl)-1-methylpyrrolidinium, 1-(3-hydroxypropyl)-1-ethylpyrrolidinium, 1-(3-hydroxypropyl)-1-propylpyrrolidinium, 1-(3-hydroxypropyl)-1-butylpyrrolidinium, 1-(4-hydroxybutyl)-1-methylpyrrolidinium, 1-(4-hydroxybutyl)-1-ethylpyrrolidinium, 1-(4-hydroxybutyl)-1-propylpyrrolidinium or 1-(4-hydroxybutyl)-1-butylpyrrolidinium, 1-(1-hydroxymethyl)-3-methylimidazolium, 1-(1-hydroxymethyl)-3-ethylimidazolium, 1-(1-hydroxymethyl)-3-propylimidazolium, 1-(1-hydroxymethyl)-3-butylimidazolium, 1-(2-hydroxyethyl)-3-methylimidazolium, 1-(2-hydroxyethyl)-3-ethylimidazolium, 1-(2-hydroxyethyl)-3-propylimidazolium, 1-(2-hydroxyethyl)-3-butylimidazolium, 1-(3-hydroxypropyl)-3-methylimidazolium, 1-(3-hydroxypropyl)-3-ethylimidazolium, 1-(3-hydroxypropyl)-3-propylimidazolium, 1-(3-hydroxypropyl)-3-butylimidazolium, 1-(4-hydroxybutyl)-3-methylimidazolium, 1-(4-hydroxybutyl)-3-ethylimidazolium, 1-(4-hydroxybutyl)-3-propylimidazolium, 1-(4-hydroxybutyl)-3-butylimidazolium, 1,3-bis(1-hydroxymethyl)-imidazolium, 1,3-bis(2-hydroxyethyl)imidazolium, 1,3-bis(3-hydroxypropyl)-imidazolium, 1,3-bis(4-hydroxybutyl)imidazolium, 1-(2-hydroxyethyl)-3-(1-hydroxymethyl)imidazolium, 1-(2-hydroxyethyl)-3-(3-hydroxypropyl)imidazolium, 1-(2-hydroxyethyl)-3-(4-hydroxybutyl)imidazollum, 1-(3-hydroxypropyl)-3-(1-hydroxymethyl)imidazolium, 1-(3-hydroxypropyl)-3-(2-hydroxyethyl)imidazolium, 1-(3-hydroxypropyl)-3-(4-hydroxybutyl)imidazolium, 1-(4-hydroxybutyl)-3-(1-hydroxymethyl)imidazolium, 1-(4-hydroxybutyl)-3-(2-hydroxyethyl)imidazolium or 1-(4-hydroxybutyl)-3-(3-hydroxypropyl)-imidazolium.

Particularly suitable cations are tetramethylammonium, trimethylalkylammonium, where the alkyl group can have 1 to 10 C atoms, trihexyltetradecylphosphonium, triisobutyl(methyl)phosphonium, tributyl(ethyl)phosphonium, tributyl(methyl)phosphonium, 1-butyl-1-methylpyrrolidinium, 1-butyl-1-ethylpyrrolidinium 1-hexyl-1-methylpyrrolidinium, 1-methyl-1-octylpyrrolidinium or 1-(2-hydroxyethyl)-3-methylimidazolium, very particularly suitable cations are 1-butyl-1-methylpyrrolidinium, 1-hexyl-1-methylpyrrolidinium, 1-methyl-1-octylpyrrolidinium or 1-(2-hydroxyethyl)-3-methylimidazolium.

Particularly suitable ionic liquids for use in the process according to the invention are 1-butyl-1-methylpyrrolidinium trifluoromethanesulfonate, 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide, 1-butyl-1-methylpyrrolidinium tris(pentafluoroethyl)trifluorophosphate, 1-hexyl-1-methylpyrrolidinium trifluoromethanesulfonate, 1-hexyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide, 1-hexyl-1-methylpyrrolidinium tris(pentafluoroethyl)trifluorophosphate, 1-methyl-1-octylpyrrolidinium trifluoromethanesulfonate, 1-methyl-1-octylpyrrolidinium bis(trifluoromethylsulfonyl)imide, 1-methyl-1-octylpyrrolidinium tris(pentafluoroethyl)trifluorophosphate, 1-(2-hydroxyethyl)-3-methylimidazolium trifluoromethanesulfonate, 1-(2-hydroxyethyl)-3-methylimidazolium bis(trifluoromethylsulfonyl)imide or 1-(2-hydroxyethyl)-3-methylimidazolium tris(pentafluoroethyl)trifluorophosphate.

In accordance with the process according to the invention, tantalum or copper ions are dissolved in a suitable ionic liquid, as described above. This can be carried out either by anodic dissolution of the metal or a suitable metal salt, for example TaH₄ or TaH₅, in the ionic liquid or by dissolution of a tantalum or copper salt in the ionic liquid.

Suitable copper or tantalum salts are, for example, copper(II), copper(I), tantalum(IV) or tantalum(V) halides, for example chlorides, bromides, iodides or fluorides, imides, for example copper(II), copper(I), tantalum(IV) or tantalum(V) bis(perfluoroalkylsulfonyl)imides, amides, for example Ta(NR₂)₄ or Ta(NR₂)₅, where R can denote an alkyl group having 1 to 4 C atoms, alkoxides, such as copper(II), copper(I), tantalum(IV) or tantalum(V) methoxide, copper(II), copper(I), tantalum(IV) or tantalum(V) ethoxide or copper(II), copper(I), tantalum(IV) or tantalum(V) tartrate. A suitable tantalum salt is also the salt TaX_(y)(bis(trifluoromethylsulfonyl)-imide)_(z), where X═F, Cl, Br or I, y=1, 2, 3 or 4, and z=1, 2, 3 or 4, and the sum y+z=4 or 5.

The salts are preferably employed in anhydrous form. However, the salts may also contain crown ethers. However, the ionic liquid comprising the copper salt may also be dried.

Particularly suitable copper or tantalum salts are salts whose anions correspond to or are chemically very similar to the anion of the ionic liquid.

In a particular embodiment, the first tantalum deposition is carried out in accordance with the invention by dissolving a tantalum salt in the ionic liquid and carrying out the second copper deposition in accordance with the invention by introducing the copper ions into the ionic liquid by anodic oxidation, thus guaranteeing freedom from water.

The presence of an alkali or alkaline earth metal fluoride in the electrochemical deposition of tantalum in accordance with the invention has proven advantageous. The fluoride should preferably be added in a ratio of 2:1 (fluodideltantalum salt) to 1:1 (fluoride/tantalum salt), preferably in the ratio 1:1.

Preferred alkali or alkaline earth metal fluorides are, for example, lithium fluoride, sodium fluoride, potassium fluoride, magnesium fluoride or calcium fluoride. Lithium fluoride is particularly preferably added.

Particularly preferred tantalum salts are tantalum tetrafluoride, tantalum pentafluoride, tantalum tetrachloride, tantalum tetrabromide, tantalum tetraiodide, tantalum pentabromide or tantalum pentaiodide. Tantalum pentafluoride is very particularly preferred.

The ion concentration in the ionic liquid for the metal deposition is preferably 10⁻⁵ to 10 mol/l. An ion concentration of 10³ to 10⁻¹ mol/l is preferably used.

For the tantalum deposition according to the invention in the presence of an alkali or alkaline earth metal fluoride, in particular lithium fluoride, an ion concentration of in each case 0.25 mol/l to 1 mol/l has proven to be the preferred range.

The metal deposition according to the invention is carried out in a protective-gas atmosphere, for example under argon, where the oxygen and water content should be below 1 ppm. The deposition is carried out in a 3-electrode cell, as is known to the person skilled in the art (for example from A. J. Bard, L. R. Faulkner, Electrochemical Methods, Wiley). In the case of the deposition of copper onto a suitable substrate, copper wires are used as counter- and reference electrodes. In the case of the deposition of tantalum, platinum wires are used as quasi-reference and counterelectrode. In general, however, any electrode material is suitable if it is ensured through the experimental set-up that the products forming at the counterelectrode do not interfere with the processes at the working electrode.

The process according to the invention is preferably carried out potentiostatically, at electrode potentials between 0 and −2000 mV vs. tantalum deposition and at temperatures between 10° C. and 350° C., preferably between 100° C. and 300° C.

However, the process according to the invention can also be carried out by means of pulsed techniques, as are known to the person skilled in the art, for example as described in J.-C. Puippe, F. Leaman, Pulse-Plating: Elektrolytische Metallabscheidung mit Pulsstrom [Pulse Plating: Electrolytic Deposition of Metal with Pulsed Current], Eugen G. Leuze Verlag, 1990.

The process according to the invention enables the metals tantalum or copper to be deposited in layer thicknesses of between 200 μm and 200 μm in continuous layers made up of micro- or nanocrystals. The desired layer thickness is controlled via the electrode potential and the charge that has flowed and the electrochemical parameters.

This correlation is described in a generally valid manner via the Faraday law:

${d = \frac{I \star t \star M}{F \star A \star \rho}},$

where F=Faraday constant, A=area, ρ=density of the metal, I=current, t=time and M=molar mass of the metal.

Ultimately, the layer thickness can be set via the current and time.

FIG. 1 shows a cyclic voltammogram of an approximately 1 molar solution of TaF₅ in 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)-imide (BMP Tf₂N) at room temperature on Au(111).

On the basis of FIG. 1, tantalum pentafluoride is apparently reduced in a number of reduction steps during the process according to the invention, If LiF is added to TaF₅/1-butyl-1-methylpyrrolidinium bis(trifluoromethyl-sulfonyl)imide, a new reduction peak arises, see FIG. 2, and tantalum is obtained, as can be seen in FIG. 3.

FIG. 2 shows a cyclic voltammogram of a 0.25 molar solution of TaF₅ and a 0.25 molar solution of LiF in 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide at 200° C. on Au(111).

FIG. 3 shows an X-ray diffraction pattern (XRD=X-ray diffraction, cobalt K alpha as X-ray radiation) of tantalum deposited from TaF₅/LiF/BMP Tf₂N [0.5 mol/l of TaF₅ and 0.5 mol/l of LiF in BMP Tf₂N] at 200° C.

FIG. 4 shows a cyclic voltammogram of 1-butyl-1-methylpyrrolidinium trifluoromethanesulfonate in which copper has previously been dissolved anodically, on Au(111). The concentration of the copper ions in the ionic liquid is 10⁻¹ mol/l.

FIG. 4A in which two successive scans are shown, shows two and three reduction processes respectively on Au(111), where the process at −1000 mV and −1700 mV can be ascribed to copper deposition. Copper is deposited in very high quality in the process according to the invention and is formed, in particular, on a nanoscale.

At higher temperatures, the same cyclic voltammograms are obtained qualitatively, but with higher current densities.

A wide variety of substrates which can be employed as cathode are possible for the electrochemical metal deposition according to the invention. The geometry of these substrates is freely selectable and is not subject to any restriction.

Suitable substrates can be selected, for example, from all categories, for example non-metals, semi-metals, metals, metal alloys, conducive or metallised ceramics or conductive or metallised plastics are possible.

A preferred non-metal is, for example, graphite.

A preferred semi-metal is, for example, silicon.

Preferred metals are, for example, gold, platinum, copper, iron, cobalt, nickel or molybdenum.

Preferred metal alloys are, for example, a very wide variety of steels or nickel alloys.

Suitable substrates can also, for example, already consist of a plurality of layers to which a further layer of tantalum or copper is applied by the process according to the invention as interlayer or final layer. The list of substrates should therefore in no way be regarded as limiting. The person skilled in the art in the relevant area of application will be able to select the suitable substrate without further information.

After deposition of tantalum and/or copper, the ionic liquid can be washed out with organic solvents or, in the case of copper, with water.

Suitable organic solvents are, for example, toluene, benzene, methylene chloride, acetonitrile, acetone, methanol, ethanol or isopropanol.

The invention also relates to a particular embodiment of the process, in which firstly tantalum in an ionic liquid comprising at least one tetraalkylammonium, tetraalkylphosphonium, 1,1-dialkylpyrrolidinium, 1-hydroxyalkyl-1-alkylpyrrolidinium, 1-hydroxyalkyl-3-alkylimidazolium or 1,3-bis(hydroxyalkyl)imidazolium cation, where the alkyl groups or the alkylene chain of the hydroxyalkyl group may each, independently of one another, have 1 to 10 C atoms, is deposited on a structured silicon chip, the tantalum ion-containing ionic liquid is replaced by pure ionic liquid with electrochemical control of the potential, the copper ion-containing ionic liquid is subsequently introduced with control of the potential, and the copper deposition is carried out.

The detailed conditions of the deposition and the suitable ionic liquids and the post-treatment of the coated substrate are revealed in the explanations described above.

Even without further comments, it is assumed that a person skilled in the art will be able to utilise the above description in the broadest scope. The preferred embodiments and examples should therefore merely be regarded as descriptive disclosure which is absolutely not limiting in any way.

EXAMPLES

The electrochemical measurements were carried out using a Princeton Applied Research (EG & G) PAR 2263 potentiostat/galvanostat.

In general, any potentiostat with or without pulse generator is suitable.

Example 1 Deposition of Tantalum from TaF₅

A saturated solution of TaF₅ and LiF in the ionic liquid 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide is prepared and transferred into the 3-electrode measurement cell under a protective-gas atmosphere at room temperature. A typical 3-electrode measurement cell was used, as described, for example, in A. J. Bard and L. R. Faulkner, Electrochemical Methods, Wiley.

The 3-electrode measurement cell has a gold electrode as working electrode (cathode), and platinum wires serve as quasi-reference and counterelectrode.

The electrode potential is set to −1300 mV vs. platinum quasi-reference.

The deposition of tantalum begins at −1250 mV. An in-situ STM photomicrograph at −1200 mV on Au(111) clearly shows (FIG. 5) that small crystallites having a height of a few nanometres are deposited. These form a layer with a thickness of about 100 nm. The metallic character can be demonstrated by current/voltage tunnel spectra (FIG. 6).

Example 2 Deposition of Tantalum from TaF₅ on Platinum

Analogously to Example 1, a 0.25 molar solution of TaF₅ and LiF in the ionic liquid 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide is prepared and transferred into the 3-electrode measurement cell under a protective-gas atmosphere at room temperature. A typical 3-electrode measurement cell was used, as described, for example, in A. J. Bard and L. R. Faulkner, Electrochemical Methods, Wiley.

The 3-electrode measurement cell has a platinum electrode as working electrode (cathode), and platinum wires serve as quasi-reference and counterelectrode.

The electrode potential is set to −1300 mV vs. platinum quasi-reference.

The deposition of tantalum begins at −1250 mV. Here too, a layer with a thickness of about 100 nm of small tantalum crystallites is deposited, as confirmed by the SEM photomicrograph (FIG. 9).

Example 3 Deposition of Copper from Copper Ion-Containing 1-butyl-1-methylpyrrolidinium trifluoromethanesulfonate

A concentration of 10⁻¹ mol/l of copper ions is set coulometrically in a 3-electrode measurement cell containing the anhydrous ionic liquid 1-butyl-1-methylpyrrolidinium trifluoromethanesulfonate.

The 3-electrode measurement cell here consists of Cu as working electrode, shaped into a coil, Cu as reference electrode and platinum as counterelectrode. The electrode potential of the copper working electrode is set to +500 mV against Cu/Cu⁺. The dissolved amount of copper ions is set via the amount of charge, which can be calculated via the formula

${c = \frac{I \star t}{F \star V}},$

where c=ion concentration, I=current, V=volume and F=96485 c/mol.

Ideally, the platinum counterelectrode is spatially separate in order to avoid re-deposition of copper thereon.

The SEM photomicrograph (FIG. 7) shows that copper is deposited on a nanoscale. A layer thickness of 10 μm was produced here. In principle, the layer thickness is unlimited, i.e. it can be produced in the desired thickness, depending on the application.

Example 4

Analogously to Example 3, copper was deposited in 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide. Copper is likewise deposited on a nanoscale in this ionic liquid, it being possible to set the layer thickness variably.

Example 5

Deposition of copper from copper(II) trifluoromethanesulfonate in 1-butyl-1-methylpyrrolidinium trifluoromethanesulfonate

In a first step, copper(II) trifluoromethanesulfonate containing water of crystallisation is dissolved in 1-butyl-1-methylpyrrolidinium trifluoromethane-sulfonate to give a 1 molar solution and transferred into a 3-electrode measurement cell under a protective gas.

The 3-electrode measurement cell has a gold electrode as working electrode (cathode), and copper wires serve as reference and counterelectrode.

The electrode potential is set to −500 mV vs. Cu/Cu⁺.

In the cyclic voltammogram in FIG. 8, 2 redox processes for the reduction and two less well separated processes for the oxidation can clearly be seen. The deposition of copper in the volume phase commences at about −1000 mV. In the region of the first reduction peak at about −250 mV, no changes on the electrode surface are evident.

Analogously to Example 3, copper was deposited on a nanoscale (<59 nm) in this ionic liquid, it being possible to set the layer thickness variably. Typically, a layer thickness of 10 μm was deposited. 

1. Process for the electrochemical deposition of tantalum and/or copper on a substrate in an ionic liquid comprising at least one tetraalkylammonium, tetraalkylphosphonium, 1,1-dialkylpyrrolidinium, 1-hydroxyalkyl-1-alkylpyrrolidinium, 1-hydroxyalkyl-3-alkylimidazolium or 1,3-bis(hydroxyalkyl)imidazolium cation, where the alkyl groups or the alkylene chain of the hydroxyalkyl group may each, independently of one another, have 1 to 10 C atoms.
 2. Process according to claim 1, characterised in that the ionic liquid has a broad electrochemical window from −2000 mV to −3500 mV against ferrocene/ferrocinium in the cathodic branch.
 3. Process according to claim 1, characterised in that the anion of the ionic liquid is selected from the group of perfluoroalkylsulfonate, perfluoroacetate, bis(fluorosulfonyl)imide, bis(perfluoroalkylsulfonyl)imide, tris(perfluoroalkyl)trifluorophosphate, bis(perfluoroalkyl)tetrafluorophosphate, tris(perfluoroalkylsulfonyl)methide or perfluoroalkylborate.
 4. Process according to claim 1, characterised in that the cation is selected from the group of linear or branched tetramethylammonium, tetraethylammonium, tetrapropylammonium, tetrabutylammonium, tetrapentylammonium, tetrahexylammonium, tetraheptylammonium, tetraoctylammonium, tetranonylammonium, tetradecylammonium, trimethylalkylammonium, trimethyl(ethyl)ammonium, triethyl(methyl)ammonium, trihexylammonium, methyl(trioctyl)ammonium, tetramethylphosphonium, tetraethylphosphonium, tetrapropylphosphonium, tetrabutylphosphonium, tetrapentylphosphonium, tetrahexylphosphonium, tetraheptylphosphonium, tetraoctylphosphonium, tetranonylphosphonium, tetradecylphosphonium, trihexyltetradecylphosphonium, triisobutyl(methyl)phosphonium, tributyl(ethyl)phosphonium, tributyl(methyl)phosphonium, 1,1-dimethylpyrrolidinium, 1-methyl-1-ethylpyrrolidinium, 1-methyl-1-propylpyrrolidinium, 1-methyl-1-butylpyrrolidinium, 1-methyl-1-pentylpyrrolidinium, 1-methyl-1-hexylpyrrolidinium, 1-methyl-1-heptylpyrrolidinium, 1-methyl-1-octylpyrrolidinium, 1-methyl-1-nonylpyrrolidinium, 1-methyl-1-decylpyrrolidinium, 1,1-diethylpyrrolidinium, 1-ethyl-1-propylpyrrolidinium, 1-ethyl-1-butylpyrrolidinium, 1-ethyl-1-pentylpyrrolidinium, 1-ethyl-1-hexylpyrrolidinium, 1-ethyl-1-heptylpyrrolidinium, 1-ethyl-1-octylpyrrolidinium, 1-ethyl-1-nonylpyrrolidinium, 1-ethyl-1-decylpyrrolidinium, 1,1-dipropylpyrrolidinium, 1-propyl-1-butylpyrrolidinium, 1-propyl-1-pentylpyrrolidinium, 1-propyl-1-hexylpyrrolidinium, 1-propyl-1-heptylpyrrolidinium, 1-propyl-1-octylpyrrolidinium, 1-propyl-1-nonylpyrrolidinium, 1-propyl-1-decylpyrrolidinium, 1,1-dibutylpyrrolidinium, 1-butyl-1-pentylpyrrolidinium, 1-butyl-1-hexylpyrrolidinium, 1-butyl-1-heptylpyrrolidinium, 1-butyl-1-octylpyrrolidinium, 1-butyl-1-nonylpyrrolidinium, 1-butyl-1-decylpyrrolidinium, 1,1-dipentylpyrrolidinium, 1-pentyl-1-hexylpyrrolidinium, 1-pentyl-1-heptylpyrrolidinium, 1-pentyl-1-octylpyrrolidinium, 1-pentyl-1-nonylpyrrolidinium, 1-pentyl-1-decylpyrrolidinium, 1,1-dihexylpyrrolidinium, 1-hexyl-1-heptylpyrrolidinium, 1-hexyl-1-octylpyrrolidinium, 1-hexyl-1-nonylpyrrolidinium, 1-hexyl-1-decylpyrrolidinium, 1,1-dihexylpyrrolidinium, 1-hexyl-1-heptylpyrrolidinium, 1-hexyl-1-octylpyrrolidinium, 1-hexyl-1-nonylpyrrolidinium, 1-hexyl-1-decylpyrrolidinium, 1,1-diheptylpyrrolidinium, 1-heptyl-1-octylpyrrolidinium, 1-heptyl-1-nonylpyrrolidinium, 1-heptyl-1-decylpyrrolidinium, 1,1-dioctylpyrrolidinium, 1-octyl-1-nonylpyrrolidinium, 1-octyl-1-decylpyrrolidinium, 1-1-dinonylpyrrolidinium, 1-nonyl-1-decylpyrrolidinium or 1,1-didecylpyrrolidinium, 1-hydroxymethyl-1-methylpyrrolidinium, 1-hydroxymethyl-1-ethylpyrrolidinium, 1-hydroxymethyl-1-propylpyrrolidinium, 1-hydroxymethyl-1-butylpyrrolidinium, 1-(2-hydroxyethyl)-1-methylpyrrolidinium, 1-(2-hydroxyethyl)-1-ethylpyrrolidinium, 1-(2-hydroxyethyl)-1-propylpyrrolidinium, 1-(2-hydroxyethyl)-1-butylpyrrolidinium, 1-(3-hydroxypropyl)-1-methylpyrrolidinium, 1-(3-hydroxypropyl)-1-ethylpyrrolidinium, 1-(3-hydroxypropyl)-1-propylpyrrolidinium, 1-(3-hydroxypropyl)-1-butylpyrrolidinium, 1-(4-hydroxybutyl)-1-methylpyrrolidinium, 1-(4-hydroxybutyl)-1-ethylpyrrolidinium, 1-(4-hydroxybutyl)-1-propylpyrrolidinium or 1-(4-hydroxybutyl)-1-butylpyrrolidinium, 1-(1-hydroxymethyl)-3-methylimidazolium, 1-(1-hydroxymethyl)-3-ethylimidazolium, 1-(1-hydroxymethyl)-3-propylimidazolium, 1-(1-hydroxymethyl)-3-butylimidazolium, 1-(2-hydroxyethyl)-3-methylimidazolium, 1-(2-hydroxyethyl)-3-ethylimidazolium, 1-(2-hydroxyethyl)-3-propylimidazolium, 1-(2-hydroxyethyl)-3-butylimidazolium, 1-(3-hydroxypropyl)-3-methylimidazolium, 1-(3-hydroxypropyl)-3-ethylimidazolium, 1-(3-hydroxypropyl)-3-propylimidazolium, 1-(3-hydroxypropyl)-3-butylimidazolium, 1-(4-hydroxybutyl)-3-methylimidazolium, 1-(4-hydroxybutyl)-3-ethylimidazolium, 1-(4-hydroxybutyl)-3-propylimidazolium, 1-(4-hydroxybutyl)-3-butylimidazolium, 1,3-bis(1-hydroxymethyl)imidazolium, 1,3-bis(2-hydroxyethyl)imidazolium, 1,3-bis(3-hydroxypropyl)imidazolium, 1,3-bis(4-hydroxybutyl)imidazolium, 1-(2-hydroxyethyl)-3-(1-hydroxymethyl)imidazolium, 1-(2-hydroxyethyl)-3-(3-hydroxypropyl)imidazolium, 1-(2-hydroxyethyl)-3-(4-hydroxybutyl)imidazolium, 1-(3-hydroxypropyl)-3-(1-hydroxymethyl)imidazolium, 1-(3-hydroxypropyl)-3-(2-hydroxyethyl)imidazolium, 1-(3-hydroxypropyl)-3-(4-hydroxybutyl)imidazolium, 1-(4-hydroxybutyl)-3-(1-hydroxymethyl)imidazolium, 1-(4-hydroxybutyl)-3-(2-hydroxyethyl)imidazolium or 1-(4-hydroxybutyl)-3-(3-hydroxypropyl)imidazolium.
 5. Process according to claim 1, characterised in that tantalum or copper ions are in dissolved form in the ionic liquid.
 6. Process according to claim 5, characterised in that the tantalum or copper ions are produced by anodic oxidation.
 7. Process according to claim 5, characterised in that the tantalum or copper ions are produced by dissolution of a tantalum or copper salt.
 8. Process according to claim 1, characterised in that an alkali or alkaline earth metal fluoride is present in the ionic liquid for the electrochemical deposition of tantalum.
 9. Process according to claim 8, characterised in that the ratio of alkali or alkaline earth metal fluoride to tantalum salt is 2:1 to 1:1.
 10. Process according to claim 1, characterised in that the substrate is a non-metal, semi-metal, metal, a metal alloy or conductive and/or metallised ceramics or a conductive and/or metallised plastic.
 11. Process according to claim 1, characterised in that, after the deposition of tantalum and/or copper, the ionic liquid is washed out with organic solvents or, in the case of copper, with water.
 12. Electrochemical process for the deposition of tantalum and subsequently copper, in which firstly tantalum in an ionic liquid comprising at least one tetraalkylammonium, tetraalkylphosphonium, 1,1-dialkylpyrrolidinium, 1-hydroxyalkyl-1-alkylpyrrolidinium, 1-hydroxyalkyl-3-alkylimidazolium or 1,3-bis(hydroxyalkyl)imidazolium cation, where the alkyl groups or the alkylene chain of the hydroxyalkyl group may each, independently of one another, have 1 to 10 C atoms, is deposited on a structured silicon chip according to claim 2, the tantalum ion-containing ionic liquid is replaced by pure ionic liquid with electrochemical control of the potential, the copper ion-containing ionic liquid is subsequently introduced with control of the potential, and the copper deposition is carried out. 